The present invention relates to centrifuges and more particularly to an improved method and apparatus for braking the rotation of a centrifuge.
In a centrifuge, the problem of decelerating or braking has always been a major factor. A braking device must perform its braking function quickly without causing a remixing of the separated components through undesired vibration and oscillation. Such factors as the mass of the device to be stopped and the speed at which it is rotating have significant effect.
In air driven centrifuges, the braking function is a much more complex problem than in spindle driven centrifuges. An air driven centrifuge, such as illustrated in U.S. Pat. No. 3,456,875 issued to George N. Hein, includes a rotor chamber having a rotor seat and a rotor having a plurality of turbine fins formed on the under side. The rotor seat includes driving air jet means for impinging pressurized air streams against the turbine flutes of the rotor for supporting and spinning the rotor on an air cushion above the rotor seat. In addition, support air jet means may also be provided within the seat for directing pressurized air streams against the under side of the rotor to support the rotor when the driving air jet streams are inactivated. Such an arrangement is disclosed in an application entitled Air Levitation For Air Driven Centrifuge filed concurrently herewith in the name of George N. Hein. When supported and spinning on the air bearing thus formed, the rotor is operating in a virtually frictionless environment.
Because air driven centrifuge rotors are supported on this substantially friction-free cushion of air, it is difficult to design a system employing air braking streams that will make the rotor come to a gradual, complete stop. While great pains in design can be taken to hold any rotational effect due to a supporting air stream to a minimum, it is difficult to completely eliminate any rotational effect. There is always a certain amount of windmilling while the supporting or holding air stream moves across the turbine flutes of such a rotor. In addition, the design of the rotor, or the loading of the sample therein, always introduces certain parameters which create critical speeds at which the rotor will precess, wobble, or vibrate excessively while decelerating. Any unbalanced force applied to the rotor for braking purposes can cause the rotor to move out of its rotational axis where it may come into contact with the sidewalls of its seat and thrash about within the centrifuge chamber.
In an application entitled Eddy Current Brake For Air Driven Centrifuge, filed concurrently herewith in the names of Douglas H. Durland, Robert J. Ehret and George N. Hein, an apparatus and method for providing a rapid and equally balanced deceleration force for a centrifuge rotor is disclosed employing eddy currents. In the preferred embodiment, a magnet means is moved between two positions. In one position, no braking effect is created in the rotating rotor by the flux field of the magnet means. In the second position the magnetic flux field causes the inducement of sufficient eddy currents in an electrically conductive portion of the rotating rotor to cause a braking effect. What is desired, is a novel apparatus for moving the magnet means between these two positions. Additionally, it would be desirable that such apparatus exhibit fail "safe" characteristics.